1. Field of the Invention
The present invention relates to an optical waveguide component incorporated in an optical communication system or an optical sensor system and a light signal processing method using the same and, more particularly, to an optical waveguide component which can provide reliable monitoring data when being operated by being incorporated in a system for monitoring the optical path or the light receiving terminal on the system subscriber side and a light signal processing method using the same.
2. Prior Art
In an optical communication system using optical fibers, an optical communication system, for example, as shown in FIG. 1 has been proposed as a system for monitoring the optical path and the light receiving terminal on the system subscriber side.
In this system, a light transmitting terminal A is installed at a station, from which signal light with a wavelength of .lambda..sub.1 is transmitted. A light coupling/separating device B is connected to the light transmitting terminal A via an optical fiber L.sub.1, and an optical time domain reflectmeter (OTDR) C is connected to the light coupling/separating device B via an optical fiber L.sub.2. To monitor a fault, a regenerating light with a wavelength of .lambda..sub.2 emitted from the OTDR C is superposed on the signal light at the light coupling/separating device B. The light signal is sent to light receiving terminals E.sub.1 and E.sub.2 on the subscriber side through optical cables L.sub.3, L.sub.5 and L.sub.4, L.sub.6 via optical waveguide components D.sub.1 and D.sub.2 having a wavelength selecting function and a reflecting function.
In each optical waveguide component D.sub.1 and D.sub.2, the transmitted light signal is separated into the signal light with a wavelength of .lambda..sub.1 and the regenerating light with a wavelength of .lambda..sub.2, and each signal light is received by each of the light receiving terminals E.sub.1 and E.sub.2 of the system subscriber.
The regenerating light with a wavelength of .lambda..sub.2 is reflected by the optical waveguide component D.sub.1 and D.sub.2, and goes back to the light coupling/separating device B on the station side through the optical cables L.sub.3 and L.sub.4, where it is separated and transmitted to the OTDR C.
The optical waveguide component D.sub.1 (D.sub.2) has a sectional construction such that, as shown in FIG. 2 and FIG. 3, which is a sectional view taken along the line III--III of FIG. 2, a lower cladding 2a and an upper cladding 2b consisting of silica glass are formed on a substrate 1 of, for example, Si single crystal, and a waveguide core 3 is embedded in the claddings.
The waveguide core and claddings can be formed by combining the flame depositing process, photolithography, and the dry etching method.
The waveguide core 3 comprises a main waveguide 3a extending in the longitudinal direction of the substrate 1 and a branch waveguide 3b branching at a predetermined angle .theta. from the main waveguide 3a at an intermediate position of the main waveguide 3a. A filter element 5 is disposed at a branch portion 4 where the branch waveguide 3b branches from the main waveguide 3a.
The filter element 5 is formed as described below.
A slit 5a with a desired depth and width is formed at the branch portion 4 by, for example, combining photolithography and the dry etching method or by using a dicing saw. Into this slit 5a, a thin film filter 5b is inserted, which has a short wavelength zone passing type dielectric thin film structure which, for example, passes a light with a wavelength of 1.3 .mu.m, 1.55 .mu.m (.lambda..sub.1) but reflects a light with a wavelength of 1.65 .mu.m (.lambda..sub.2). A thin film filter 5b is fixed into the slit 5a with an optical adhesive 5c such as optical epoxy resin.
At this time, the filter element 5 is designed so that the path of light of wavelength .lambda..sub.2 can be surely changed to the branch waveguide 3b by forming at a predetermined angle with respect to the light axis direction of the main waveguide 3a.
On the other hand, on the branch waveguide 3b, a plurality of (four in the figure) reflecting elements 6a, 6b, 6c, and 6d are arranged in the extending direction of the branch waveguide 3b.
These reflecting elements 6a, 6b, 6c, and 6d consist of slits engraved at equal intervals at right angles to the light axis direction of the branch waveguide 3b usually by combining photolithography and the dry etching method. They are filled with air. The slits may be filled with a substance having a different refractive index from that of the waveguide core composing the branch waveguide 3b, such as silicone resin.
Therefore, as shown in FIG. 4, which is a partially enlarged view of FIG. 2, the reflecting element has a rectangular shape transversing the branch waveguide 3b when viewed in plan. The reflecting element has two faces of S.sub.1 and S.sub.2. Each of these faces S.sub.1 and S.sub.2 is at right angles to the extending direction (light axis direction) of the branch waveguide 3b.
In the optical waveguide component D.sub.1 (D.sub.2), as shown in FIG. 2, an input side optical fiber 7a and an output side optical fiber 7b are connected to the main waveguide 3a. A light in which regenerating light with a wavelength of .lambda..sub.2 is superposed on signal light with a wavelength of .lambda..sub.1 is inputted from the input side optical fiber 7a.
The light goes through the main waveguide 3a and reaches the filter element 5. At the filter element 5, the signal light with a wavelength of .lambda..sub.1 (1.3 to 1.55 .mu.m) passes through the filter element 5 and goes through the main waveguide 3a on the output side, being sent to the light receiving terminal (not shown) of the system subscriber through the output side optical fiber 7b.
On the other hand, the regenerating light with a wavelength of .lambda..sub.2 (1.65 .mu.m) is reflected by the filter element 5, and goes through the branch waveguide 3b by changing its light path.
The regenerating light with a wavelength of .lambda..sub.2 reaches the first reflecting element 6a. Part of the regenerating light is reflected by the face S.sub.1 of the reflecting element 6a, and the remaining regenerating light passes through the reflecting element 6a, reaching the face S.sub.2 of the reflecting element 6a. The part of the regenerating light reflected by the face S.sub.1 goes back toward the filter element 5 along the light axis direction of the branch waveguide 3b as indicated by the arrow q.sub.1 in FIG. 4. It is reflected by the filter element 5 again, and goes back to the system on the station side through the main waveguide 3a via the input side optical fiber 7a as a return light.
On the other hand, for the remaining regenerating light passing through the reflecting element 6a and reaching the face S.sub.2 of the reflecting element 6a, part of it is reflected by the face S.sub.2 as indicated by the arrow q.sub.2, and goes back to the station side system through the branch waveguide 3b, the filter element 5, and the main waveguide 3a like the light reflected by the face S.sub.1. The remaining regenerating light passing through the face S.sub.2 of the reflecting element 6a goes through the branch waveguide 3b and reaches the next reflecting element 6b, where part of it goes back and the remaining light goes to the next reflecting element 6c like the case of the reflecting element 6a.
Thus, the regenerating light with a wavelength of .lambda..sub.2, which is reflected by the filter element 5 and whose light path is changed to the branch waveguide 3b, is returned to the station side system as a return light while being subjected to reflection and transmission at the reflecting elements in sequence.
This return light goes back to the light coupling/separating device B shown in FIG. 1, where the light is separated and inputted to the OTDR C. Then, time-series signal processing is performed by a signal processing mechanism in the OTDR C.
This time-series signal processing is performed as described below. The processing will be explained for the case where the optical waveguide component D.sub.1 in FIG. 1 has four reflecting elements disposed at equal intervals, and the optical waveguide component D.sub.2 has three reflecting elements disposed at equal intervals.
First, in the optical waveguide component D.sub.1, the return time of each return light going back to the OTDR C by being reflected by respective reflecting elements 6a, 6b, 6c, and 6d is converted into a distance from the OTDR C to each reflecting element. This distance and the light power of return light are monitored, and the light power of the reflected light at each reflecting element is obtained as a monitoring signal. One example of the obtained monitoring signal is shown in FIG. 5.
In FIG. 5, an arbitrary point a.sub.1 is set to a reference point. A point a.sub.2 at a distance of (2.times.F .mu.m) from the reference point a.sub.1 is a read start point of the read signal. The presence of light power is detected every F .mu.m from the read start point a.sub.2. Digital conversion is performed; the presence of light power (with reflection) is converted into 1, and the absence of it (without reflection) into 0. Thus, the bit pattern of the read signal is judged.
For example, in the case of the monitoring signal shown in FIG. 5, the bit pattern is judged to be [10101010]. That is to say, the bit pattern of the optical waveguide component D.sub.1 is recognized as [10101010].
This time-series signal processing is performed in the same way for the optical waveguide component D.sub.2. Since the optical waveguide component D.sub.2 has three reflecting elements, the bit pattern thereof is assumed to be recognized as [10101000].
The OTDR C discriminates between the return light (read signal) from the optical waveguide component D.sub.1 and the return light (read signal) from the optical waveguide component D.sub.2 by identifying the difference between the bit patterns of the optical waveguide components D.sub.1 and D.sub.2.
When there is no fault such as a broken line on the light path, by the above-described time series signal processing, the OTDR C can usually detect each of the bit pattern return light from the optical waveguide component D.sub.1 and the bit pattern of the return light from the optical waveguide component D.sub.2, respectively.
However, if there is any fault such as a broken line on the side of the optical cable L.sub.3 in FIG. 1, the return light from the optical waveguide component D.sub.1 is not inputted to the OTDR C. Therefore, the bit pattern recognized by the OTDR C is only [10101000] based on the return light from the optical waveguide component D.sub.2. Inversely, if there is any fault such as a broken line on the optical cable L.sub.4, the bit pattern recognized by the OTDR C is [10101010] only.
Thus, in the optical communication system shown in FIG. 1, by performing the time-series signal processing of return light inputted to the OTDR C, the presence of broken line or other faults on the optical cables L.sub.3 and L.sub.4 can be determined, and which of the optical cables has a fault point can be determined.
Further, by changing the number of reflecting elements arranged on the branch waveguide, an optical waveguide component which generates a bit pattern different for each light receiving terminal of system subscriber can be manufactured, and incorporated in each light receiving terminal side. Also, by performing the detection of return light by the OTDR C and the time-series signal processing, the presence of a fault point of the light path connected between the station side and the system subscriber side and the occurrence position of the fault point can be determined.
For the conventional optical waveguide component, as described above, the reflecting element arranged on the branch waveguide has a rectangular shape at right angles to the branch waveguide when viewed in plan, and the reflecting elements are arranged at equal intervals. This poses the following problems.
First, as shown in FIG. 4, the regenerating light with a wavelength of .lambda..sub.2 going through the branch waveguide 3b by changing its light path at the filter element 5 is reflected by two faces of S.sub.1 and S.sub.2 which are parallel with each other to form a return light. Therefore, the return lights reflected by these faces interfere with each other when returning to the OTDR C.
If such a problem occurs, the light power monitoring signal as shown in FIG. 5 cannot be detected, so that when the time-series signal processing is performed on the basis of the obtained monitoring signal, the bit pattern is mistakenly recognized inevitably.
When the interval between the reflecting elements and the width of the reflecting element itself have a substantially equal length, that is, the intervals between all the faces S.sub.1 and S.sub.2 are substantially equal, the number of the arranged reflecting elements are recognized mistakenly, so that it is difficult to accurately identify the bit pattern by means of the OTDR.
Further, there are the following problems.
For example, when two reflecting elements are arranged so that the bit pattern of return light from the optical waveguide component D.sub.1 in FIG. 1 is [1010] and the bit pattern of return light from the optical waveguide D.sub.2 is [0101], the monitoring signal shown in FIG. 6 should be obtained in the OTDR C for the former return light, and the monitoring signal shown in FIG. 7 should be obtained for the latter return light.
Since the reference point for digital conversion of the monitoring signal is not specified, in FIG. 6, if a position F.sub.1 is set to the reference point a.sub.1, and the position F.sub.2 at a distance of 2.times.F from the reference point a.sub.1 is set to the read start point a.sub.2 of the read signal, digital conversion of the monitoring signal is performed as the bit pattern [1010]. However, in FIG. 6, if a position F.sub.1 ' is set to the reference point, and the position F.sub.2 ' at a distance of 2.times.F from the reference point is set to the read start point of the read signal, the bit pattern obtained in the digital conversion of the monitoring signal becomes [0101].
In FIG. 7, if a position F.sub.3 is set to the reference point a.sub.1, and the position F.sub.4 at a distance of 2.times.F from the reference point a.sub.1 is set to the read start point a.sub.2 of the read signal, digital conversion of the monitoring signal is performed as the bit pattern [0101]. However, if the digital conversion of the monitoring signal is performed with the position F.sub.3 ' being set to the reference point a.sub.1, the obtained bit pattern becomes [1010]. In this case, the bit pattern is the same as that obtained by the digital conversion of return light from the optical waveguide component D.sub.1 with the position F.sub.1 being set to the reference point all as shown in FIG. 6.
As described above, the bit pattern of return light differs depending on where the reference point a.sub.1 is set for both the optical waveguides D.sub.1 and D.sub.2. Moreover, in the OTDR, the bit pattern [1010] and the bit pattern [0101] cannot be distinguished from each other, so that the time-series signal processing of return light from each optical waveguide component cannot sometimes be performed accurately.
Further, when the reflecting elements are arranged at equal intervals on the branch waveguide, multiple reflection occurs between the reflecting elements. Therefore, ghost information is sometimes included in the information obtained by the OTDR, resulting in erroneous recognition of data.
FIG. 8 shows a reflected waveform of return light in the case where eight reflecting elements are disposed at right angles to the light axis direction of the branch waveguide 3b in the optical waveguide component D.sub.1 shown in FIG. 2 and regenerating light with a wavelength of (.lambda..sub.2 =1.65) .mu.m is superposed on signal light with a wavelength of (.lambda..sub.1 =1.55) .mu.m. This optical waveguide component D.sub.1 has a 50 .mu.m thick cladding, made of SiO.sub.2, formed on a Si single crystalline substrate 1 with a length of 15 mm, a width of 5 mm, and a thickness of 1 mm. In the cladding is embedded a waveguide core 3 made of SiO.sub.2 -TiO.sub.2 and having a path width of 8 .mu.m, a path height of 8 .mu.m, and a specific refractive index difference .DELTA.: 0.3%. At the branch portion 4, a filter element 5 is formed, in which a short wavelength passing type thin film filter 5b, which passes a light with wavelengths of 1.3 .mu.m and 1.55 .mu.m through a 25 .mu.m wide slit 5a and reflects a light with a wavelength of 1.65 .mu.m, is inserted and bonded with an optical adhesive 5c. In the optical waveguide component D.sub.1, eight reflecting elements consisting of a 50 .mu.m wide transverse slit (filled with nothing: vacant filled with air) are arranged at equal intervals of 200 .mu.m on the branch waveguide 3b.
In FIG. 8, the first peak is a signal for the return light reflected by the filter element 5. As seen from FIG. 8, ten reflected waveforms are observed though eight reflecting elements (bit number 8) are arranged. This is because since the reflecting elements are arranged at equal intervals, and the refractive index difference between the waveguide core and the air is large, multiple reflections with high reflection power occur between the reflecting elements, resulting in mixing of a ghost signal. From this reflected waveform pattern, the ghost signal cannot be identified.
Since the reflecting elements of a rectangular shape in plan view are arranged at equal intervals at right angles to the branch waveguide on the waveguide, erroneous recognition of monitoring data occurs, so that the facilities on the system subscriber side cannot be monitored accurately.
In the case of the above-described optical waveguide component, when the component is manufactured, it is difficult to make certain what degree of loss occurs in the filter elements and the reflecting elements in forming them.
Even when a loss is found in detecting the monitoring signal of return light, it is impossible to determine whether the loss is based on the filter element or the reflecting elements or it is based on the abnormality loss of optical fiber connected to the optical waveguide component.